1. Field of the Invention
The present invention relates to a circuit and a method for driving an ultrasonic motor. More particularly, this invention pertains to a circuit and a method for controlling a voltage to be applied to the ultrasonic motor's piezoelectric element.
2. Description of the Related Art
One conventional ultrasonic motor design includes a stator provided with a piezoelectric element divided into two portions, and a rotor supported on and pressed against the stator such that the rotor is rotatable. In this ultrasonic motor, high-frequency voltages of different phases are applied to the individual divided portions of the piezoelectric element. The voltages are applied such that they generate a progressive wave on the surface of the stator to thereby rotate the rotor. To drive the ultrasonic motor, the frequency of the applied voltage should be equal to the resonant frequency of the motor. The resonant frequency fluctuates with changes in the environmental conditions, such as temperature, and with changes in the load acting on the output shaft of the motor. In order to stably drive the motor regardless of this frequency fluctuation, it is known to provide the ultrasonic motor with an automatic follow-up circuit as disclosed in Japanese Unexamined Patent Publication No. Sho 62-203575.
As shown in FIG. 8, the ultrasonic motor disclosed in this Japanese patent document has a monitor electrode 42 provided on a piezoelectric element 41 to monitor the voltage induced by the excitation of the piezoelectric element 41. After this monitor signal is smoothed by a smoothing circuit 43, its DC component is applied to a comparator 44. The comparator 44 compares the DC component of the monitor signal with a predetermined reference voltage and outputs a predetermined voltage in accordance with the comparison result. A voltage controlled oscillator (VCO) 45 controls the frequency of a source voltage in accordance with the output voltage of the comparator 44. The source voltage is applied via a phase shifter (PS) 46 and a power amplifier 47 to the piezoelectric element 41.
The output of the motor and the drive frequency have the relationship as shown in FIG. 9, which shows that a high output is obtained in the proximity of the resonant frequency where noise is likely to occur.
Therefore, in the circuit shown in FIG. 8, when the reference voltage of the comparator 44 is set high to provide a high output, the frequency of the source voltage may fall into a frequency range where noise occurs due to the environmental and load based frequency fluctuations. In this case, noise will occur. If the reference voltage is set low to prevent the generation of noise under any environmental condition, a high output will not be attained. In short, the conventional ultrasonic motor has the shortcoming that a high output cannot be provided without generating noise.